1. Field of the Invention
The present invention relates to ultrasonic sealers for packaging machines. Specifically, the present invention relates to an ultrasonic top sealer for a gable top carton packaging machine.
2. Description of the Related Art
Gabled containers made from paperboard material abound in many different environments. These containers are used commonly for both edible and non-edible products.
Typically, this type of container is erected from a pre-formed paperboard blank which is coated on both sides with polyethylene, or other known heat-sealable material. The blank has pre-formed fold lines which allow sequential folding of parts of the blank in a predetermined fashion to erect the container. In one preliminary folding stage, the container assumes a cup shape with an open end. A closure portion, which is made up of gable panels and a plurality of fins, is then reconfigured so that the panels converge upwardly to the fins which are brought into mutually overlying relationship. The closure portion of the container can be sealed using a number of different techniques, among which is an ultrasonic sealing technique. Ultrasonic sealing is carried out by sandwiching the fins between a sealing horn/sonotrode and anvil. Vibrational energy generated through the horn/sonotrode causes a fusion between the heat-sealable material on abutting fin surfaces. Exemplary ultrasonic sealing apparatus and methods are described in each of U.S. Pat. Nos. 5,564,255; 5,575,884; and 5,605,026, all of which have been assigned to Tetra Laval Holdings and Finance S.A.
For product that is required to be maintained in an aseptic environment, hermetic sealing between the fins is required. Conventional hot air sealing techniques may not be capable of effecting the required quality of hermetic seal for these products. Thus, ultrasonic sealing is conventionally used in most such operations. While conventional ultrasonic sealing techniques may be practiced to produce a high integrity hermetic seal, conventional ultrasonic sealing techniques introduce a time factor that must be contended with.
More particularly, the heat-sealable material must be cooled after fusion sufficiently that it will stably maintain the seal. The seal is put under stress by the restoring force in the gable panels and fins after the captive force between the sealing horn/sonotrode and anvil is eliminated. This problem can be minimized by allowing additional cooling time. However, doing so may not be compatible with the intended operating speeds of modern filling equipment. As an example, a line which is designed to operate at 14,000 packages per hour, allows only approximately 580 ms for each sealing step. The objectives of making a high integrity hermetic seal and producing the seal quickly on a filling line thus come into competition. By effecting the seal over substantially the entirety of the exposed fin surfaces, a more aggressive holding force can be established between the fins. However, the larger the area that is sealed, the greater is the heat retention within the seal and the longer becomes the cooling time for the seal to be stabilized.
One major problem with the use of ultrasonic sealing is the precise alignment of the sonotrode and anvil relative to each other and relative to the carton pathway. Due to the tremendous forces and vibrations involved in ultrasonic sealing, the unit is usually tightly restrained on the packaging machine which creates a problem when adjustments are necessary. These adjustments may arise when new carton sizes are used, or new material (thicker or thinner) are used, or during cleaning of the packaging machine. Further, the chain used in conveying cartons is stretched over its lifetime approximately five millimeters. This stretching of the chain requires adjustments to the ultrasonic sealing device to precisely align the anvil an sonotrode with the top fin of a carton.